Question A four footed animal from the waist down

Are centaur build mutants more likely to burn out or have their bodies give out as it is changing, thus why we've only really seen so far magic our shapeshifting, or spirit manipulated 'taurs?

Well, it's a harder sell to present it strictly as a mutation issue, with no magic/avatar involved. We are talking a mutation that not only borrows traits from other animals, but also does some major readjustments, like going from four limbs to six, integrating two redundant sets of internal organs... all for a *single*, *spontaneous* mutation event?
Having some external factor directing the changes, be it magic or a mad scientist, helps to make it a bit more believable.

It may be like the more extreme GSD cases: the body has to remodel the spine, add limbs, and come up with another body's worth of weight, but none of the intermediate configurations are safe for life.

A major hurdle is having the spine permanently bent ninety degrees, without any of the musculature needed to hold the upper body upright ( they lost much of that with the human legs and pelvis ) is a bad design. In hardly any time at all, what we end up with is a teenager who's shunned for being a mutant and a freak, paralyzed from the waist back, and yet still in constant back pain from slipped disks or similar.

I'm having my doubts that the digestive system won't be even more of a problem.

A somewhat more functional body plan might be to base it on a giraffe or similar herbivore with a forward leaning, body-length neck. The second pair of shoulders and would be partway up the neck, with the humanoid "torso" only being the bones and musculature needed to support adding two limbs. Even that's going to be "heavy" GSD.

It seems reasonable that the more extreme the physical change/GSD, the greater the chance of death during or soon after manifestation. However as we rarely show mutants who die as a result, they don't appear at Whateley

The giraffe solution makes the most sense to me. The redun